Surface acoustic wave (SAW) devices are much used today in electronic communication, especially SAW devices arranged to provide filtering functions. Filters formed from SAW devices are particularly useful in connection with portable radio and telephones. Such radios telephones operate typically in the 500-1500 MHz range and higher.
Filter characteristics that are of particular interest to the radio designer are: (a) the pass-bandwidth, that is, the range of frequencies within which the filter passes a signal with acceptable loss, (b) the pass-band attenuation, that is, how much loss occurs in the pass-band, (c) the transition bandwidth, that is, the range of frequencies which separate the pass-band and the stop-band, and (d) the stop-band attenuation, that is, the attenuation outside the pass-band and transition-band where no signal is desired to be transmitted. The stop band is the frequency region in which the filter provides very high attenuation. The transition band is the frequency region in which the attenuation rapidly increases from a low value (little attenuation) at the corner frequency of the pass-band to a high value (large attenuation) in the stop-band. The transition band is also referred to in the art as the "skirt" of the filter transfer characteristic, e.g., the fall-off region on either side of the pass-band on a plot of filter attenuation versus frequency.
It is a feature of SAW filters that the widths of the pass-band and transition-band are related. The choice and cut of piezoelectric substrate material from which the SAW filter is constructed and the electrode shape, spacing and location influence the characteristics of the SAW filter. Design modifications which cause the pass-band to increase in width generally also cause the transition-bandwidth to increase. Conversely, those choices which allow one to obtain small transition-bandwidths also yield narrow pass-bandwidths. This makes it extremely difficult to design SAW filters which simultaneously provide a relatively wide pass-bandwidth but at the same time have very narrow transition-bandwidths, that is, steep skirts on the filter transfer characteristic.
It is an advantage of the present invention that it overcomes these limitations of the prior art so that a greater pass-bandwidth is obtained while preserving a narrow transition-bandwidth (steep skirt) on at least one side of the pass-band. Such a SAW filter is advantageously used to improve the properties of radios.